Nitride semiconductor devices have been extensively developed as semiconductor devices with high withstand voltage and output by utilizing their characteristics such as a high saturated electron velocity and a wide band gap. Many reports have been made regarding field-effect transistors, especially high electron mobility transistors (HEMTs). AlGaN/GaN HEMT, in particular, has been receiving much attention, with GaN as an electron transport layer and AlGaN as an electron supply layer. AlGaN/GaN HEMT has a strain at a layer including AlGaN caused by a difference in lattice constants between GaN and AlGaN. The piezoelectric polarization and spontaneous polarization of AlGaN are induced thereby, thus generating two-dimensional electron gas (2DEG) at high concentration. Accordingly, high withstand voltage and output can be achieved.
Related techniques are disclosed in Japanese Unexamined Patent Application Publication Nos. 2007-329483, 2005-86171, and 2006-278857.